Dengue and Japanese encephalitis viruses are important causes of human disease on a global scale and vaccine development against both viruses remains a public health priority. Based on the efficacy of the live- attenuated viral vaccine against yellow fever virus, the potential of expressing the structural proteins of Dengue and Japanese encephalitis viruses in the context of chimeric yellow fiver viruses is being investigated. Aim 1 will determine whether an engineered chimeric virus can provide protective immunity against challenge with neuropathogenic Japanese encephalitis virus in adult mice, and to compare relative immunogenicity and protection with available Japanese encephalitis vaccine products. Aim 2 will fully characterize a chimeric yellow fever virus expressing the envelope protein of a dengue type 2 strain, and determine whether this novel virus can elicit protection in the mouse model. Aim 3 will use the chimeric yellow fever virus system to define the genetic determinants within the Japanese encephalitis virus envelope protein, which differentiate a highly neurovirulent clone from a nonvirulent clone. Aim 4 will determine whether a profound attenuation of the YS/JE-SA, 4-14-2 chimeric virus by investigating the effects of the envelope protein on different stages of viral replication. The overall goal of this proposal is to evaluate whether this chimeric virus technology warrants further consideration for development of human live- attenuated vaccines for dengue, Japanese encephalitis, and perhaps other medically important flaviviruses and to provide better knowledge of the molecular pathogenesis of these viruses.